With the advent of higher utility rates, power factor penalties and demand charges, prior art induction motors have many disadvantages. Most induction motors in use are over-sized and inefficient. Consequently power bills are higher than need be, due to motor inefficiency, high demand and poor power factor (KW/KVA). As is known, the power factor involves the phase relationship between the ac voltage and the ac current. Utility companies generally charge a premium to the user when the power factor falls below 0.85 (a power factor of unity is present when the voltage and current are of the same waveform completely in phase).
When energy rates were low, these drawbacks were not as important as they now are. Often demand (the total electrical power that needs to be available, but not necessarily used from the line) and power factor penalties are as much or more than the basic energy charge.
The most efficient prior art, single-phase induction motors are of the permanent-split capacitor design, but they have low torque characteristics and are efficient only when the magnetic field of the direct phase winding is balanced with that of the auxiliary phase winding and their respective currents are displaced by 90.degree.. In most split capacitor motors, a large stator winding is directly connected to the power terminals and a smaller auxiliary winding, serially connected to a capacitor, is also connected across the input. The 90.degree. displacement of current between both stator windings only exists at design load; at other load points a disproportionate distribution of magnetic flux exists which sets up negative sequence currents in the rotor and stator, space harmonics in the air gap (e.g., the degree to which the flux distribution in the air gap is not sinusoidal) and high leakage reactance from the stator end turns. For example, an imbalance of phase voltages on the order of 3% can cause a 15% to 20% increase in motor losses.
This condition is not restricted to single-phase motors but is also prevalent in polyphase motors when an imbalance occurs in the polyphase voltage supply. These losses in both single and polyphase motors can degrade insulation and reduce bearing life due to overheating of the rotor and, in addition to overheating, an imbalance creates higher magnetostriction noise and poor operating performance, as can be seen in Table 1.
Another significant disadvantage is in the manufacture of new motors. Engineers are now focusing on design tolerances in an attempt to increase motor efficiencies, producing a motor which is more susceptible to failure due to environmental changes and bearing wear. Attempts have been made to create a balanced condition by a series resonating winding in combination with a phase winding but this is a tuned condition for a narrow spectrum only and at certain load points circulating harmonic currents increase and the efficiency is reduced to below that of the standard design.
Induction motors and generators are efficient only when properly sized to the load and when the line voltage is balanced. When operated below design load or with a system imbalance, a disproportionate polar magnetic condition exists which sets up negative sequence currents in the rotor and stator, space harmonics in the air gap and high leakage reactance due to high currents in the phase winding. Again, an imbalance in the order of 3% can cause a 15% to 20% increase in motor or generator losses. This reduces insulation and bearing life and creates an imbalance which is manifested as higher magnetostriction noise and poor operating performance. Attempts have also been made to create a balanced and controlled condition in the motor by a series resonating winding in combination with a phase winding but this is a tuned condition for a narrow spectrum and at certain load points circulating harmonic currents increase and the efficiency is reduced to below that of the standard design.